5:15 PM - 6:45 PM
[SCG44-P10] Experimental study on rheological property of pelitic schists and its relationship to the Nankai Trough seismogenic zone.
Various activities of slow earthquakes have been observed near the upper and lower limits of the plate boundary seismogenic zone of the Nankai Trough. Since the Nankai Trough is a subduction zone composed of accretionary complex, behaviors of the plate boundary faults in slow/fast seismogenic zones can be controlled by low strength pelitic rocks. In this study, I conducted deformation experiments oon pelitic rocks from Sanbagawa metamorphic belt, an older equivalent of the presently active Nankai accretionary complex, to understand their rheological properties their relationship to the Nankai Trough seismogenic zone.
We conducted shear deformation experiment using Griggs-type apparatus. The sample was a pelitic schist collected from the Sanbagawa metamorphic belt, Shikoku Island, Japan. XRD measurements indicate that the pelitic schist is mainly composed of quartz (~55wt%) and muscovite (~10wt%). The deformation experiments were conducted under the temperature of 480℃ and the confining pressure (Pc) of 1250 MPa, which condition is equivalent to the peak metamorphic condition of the pelitic schist. These temperature and pressure conditions are also corresponding to the environment of the ETS region at the Nankai Trough. In order to investigate the effect of water on the mechanical behavior of pelitic schists, experiments were conducted under three hydrous conditions: dry condition, wet (undrain) condition, and drain condition under a constant pore pressure (Pp) of ~400 MPa. The shear rate was 0.23 μm/s in all experiments, which rate is close to the slip velocity of short-term sloe slip events.
The experimental results showed that, in all conditions, the strength of the pelitic schist was lower than that of quartz and serpentinite in similar experimental conditions. After reaching peak strength, unstable slip was observed. Especially in the dry condition, stick-slip with audible sound was observed. The friction μ of the pelitic schist at the dry condition is closer to that of dry muscovite (μ ~0.4), indicating that the strength is dominantly controlled by muscovite rather than quartz (μ ~0.6). The friction coefficients of the pelitic schist at the wet and drain conditions were close to that pf wet muscovite (μ ~0.35), indicating that the strength is further reduced by the presence of water. Tokle et al. (2023) indicate that the strength of quartz-muscovite aggregates is reduced due to the preferential reaction and deformation of the muscovite with water. The stress drop rates were 4.15-17.9 MPa/s under wet and drain conditions. On the other hand, the stress drop rate under the dry condition was 243 MPa/s. This value may be underestimated due to the low time resolution (1 Hz) of the mechanical data, and the stress drop rate at dry conditions may be orders of magnitude higher than that recorded in this study. Assuming the stiffness of the Griggs apparatus is 2000 MPa/mm, the sliding velocity was 2.1 -9.0 μm/s under wet and drain conditions, and 121.5 μm/s or even higher under dry condition.
Experimental results demonstrated that the pelitic schist containing muscovite exhibits unstable behaviors with lower strength in the wet condition. The behavior of the ETS region of the Nankai Trough may be strongly influenced by the complex slip behaviors of pelitic schist in addition to the presence of water, which also reduces the strength. While the deformation experiments under high pressure showed that the presence of water reduced the strength and changed the mechanical behavior, no effect of high pore pressure was observed in this study. Future drain experiments under high pore pressure should be conducted to investigate the effect of pore pressure on the strength of rocks under high pressure in more detail.
We conducted shear deformation experiment using Griggs-type apparatus. The sample was a pelitic schist collected from the Sanbagawa metamorphic belt, Shikoku Island, Japan. XRD measurements indicate that the pelitic schist is mainly composed of quartz (~55wt%) and muscovite (~10wt%). The deformation experiments were conducted under the temperature of 480℃ and the confining pressure (Pc) of 1250 MPa, which condition is equivalent to the peak metamorphic condition of the pelitic schist. These temperature and pressure conditions are also corresponding to the environment of the ETS region at the Nankai Trough. In order to investigate the effect of water on the mechanical behavior of pelitic schists, experiments were conducted under three hydrous conditions: dry condition, wet (undrain) condition, and drain condition under a constant pore pressure (Pp) of ~400 MPa. The shear rate was 0.23 μm/s in all experiments, which rate is close to the slip velocity of short-term sloe slip events.
The experimental results showed that, in all conditions, the strength of the pelitic schist was lower than that of quartz and serpentinite in similar experimental conditions. After reaching peak strength, unstable slip was observed. Especially in the dry condition, stick-slip with audible sound was observed. The friction μ of the pelitic schist at the dry condition is closer to that of dry muscovite (μ ~0.4), indicating that the strength is dominantly controlled by muscovite rather than quartz (μ ~0.6). The friction coefficients of the pelitic schist at the wet and drain conditions were close to that pf wet muscovite (μ ~0.35), indicating that the strength is further reduced by the presence of water. Tokle et al. (2023) indicate that the strength of quartz-muscovite aggregates is reduced due to the preferential reaction and deformation of the muscovite with water. The stress drop rates were 4.15-17.9 MPa/s under wet and drain conditions. On the other hand, the stress drop rate under the dry condition was 243 MPa/s. This value may be underestimated due to the low time resolution (1 Hz) of the mechanical data, and the stress drop rate at dry conditions may be orders of magnitude higher than that recorded in this study. Assuming the stiffness of the Griggs apparatus is 2000 MPa/mm, the sliding velocity was 2.1 -9.0 μm/s under wet and drain conditions, and 121.5 μm/s or even higher under dry condition.
Experimental results demonstrated that the pelitic schist containing muscovite exhibits unstable behaviors with lower strength in the wet condition. The behavior of the ETS region of the Nankai Trough may be strongly influenced by the complex slip behaviors of pelitic schist in addition to the presence of water, which also reduces the strength. While the deformation experiments under high pressure showed that the presence of water reduced the strength and changed the mechanical behavior, no effect of high pore pressure was observed in this study. Future drain experiments under high pore pressure should be conducted to investigate the effect of pore pressure on the strength of rocks under high pressure in more detail.